FIGS. 5 and 6 show constructions of typical solenoids that are conventionally known.
A solenoid 10 includes an excitation coil 12, a yoke 14 that is assembled so as to surround the excitation coil 12, a bearing 15 disposed in a central part of the excitation coil 12, and a slider 16 (a moving iron core or plunger) that is guided in a sliding state by the bearing 15 (see FIGS. 1 and 2, etc., of Japanese Laid-Open Patent Publication No. H05-211744).
The yoke 14 is constructed of at least two members, an upper yoke 14a and a lower yoke 14b, with the upper yoke 14a being disposed at one end and the lower yoke 14b being provided so as to close the other end of an enclosure 19 for the slider 16 so as to limit the movement in a direction A of the slider 16.
A surface 14c of the lower yoke 14b facing the other end-end surface 16a of the slider 16 functions as a fixed iron core.
When current flows through the excitation coil 12 of the solenoid 10 shown in FIG. 5, a magnetic path a is formed as shown by the dashed line, for example. It should be noted that the direction of the magnetic path a shown here is merely an example.
The magnetic path a passes inside the yoke 14, enters into the slider 16 from the upper yoke 14a, moves through the slider 16 along the axial direction toward the lower yoke 14b side, and passes to the fixed iron core part 14c of the lower yoke 14b from the other end-end surface 16a of the slider 16 via the air. Then the magnetic path a passes from the lower yoke 14b to the upper yoke 14a so as to form a closed loop.
The slider 16 is pulled onto the fixed iron core part 14c by the magnetic force produced in the gap B between the other end-end surface 16a of the slider 16 and the fixed iron core part 14c of the lower yoke. This magnetic force is the propulsion of the solenoid.
The propulsion of the solenoid 10 decreases exponentially in accordance with the distance of the gap B (that is, the stroke).
The construction of another conventional solenoid is shown in FIG. 6. Here, components that are the same as in the construction of the solenoid shown in FIG. 5 have been assigned the same reference numerals and description thereof has been omitted.
In this solenoid 20 also, the lower yoke 14b is provided so as to cover the other end-side end of the enclosure 19 for the slider 16. The fixed iron core part 14c of the lower yoke 14b is provided so as to protrude into the enclosure 19 for the slider 16, and a front end of the fixed iron core part 14c is formed as a concave 17 that is hollow in accordance with the shape of the other end-end surface 16a of the slider 16.
The other end-end surface 16a of the slider 16 is formed with a sharpened front end where the radius gradually decreases toward the other end-side so as to be capable of being enclosed in the concave 17 formed in the front end of the fixed iron core part 14c (see FIG. 1 of Japanese Laid-Open Patent Publication No. H07-336943).
The magnetic path in this solenoid 20 forms the same route as the magnetic path of the solenoid 10 shown in FIG. 5, and therefore is not illustrated, with the propulsion of the solenoid 20 being generated by a gap between the fixed iron core part 14c and the other end-end surface 16a of the slider 16. It is also known that the propulsion-displacement characteristics change in accordance with the taper angle of the other end-end surface 16a of the slider 16 in the solenoid 20.
As described above, the propulsion of the solenoid is determined by the magnitude of the magnetic energy stored in the gap between the fixed iron core and the slider. That is, magnitude of the propulsion is determined by the distance between the fixed iron core and the slider.
Here, the relationship between the stroke (amount of displacement) of the slider and the generated propulsion in a conventional solenoid is shown in FIG. 7. As shown in FIG. 7, in a conventional solenoid, the propulsion is smallest at a position where the slider is furthest away from the fixed iron core part and the propulsion increases as the slider approaches the fixed iron core part.
However, when the movable range of the slider and the controlled range (the operation range) have the relationship shown in FIG. 7, it is not possible to use large propulsion within the controlled range through which control of the solenoid is actually desired. The propulsion characteristics are also nonlinear, which means the controllability is poor.
In this kind of conventional solenoid, propulsion is generated between end surfaces of the slider at the end of the movable range and the fixed iron core part, and there has been the problem that as the movable range becomes wider, it has not been possible to set the controlled range at the optimal range in the propulsion characteristics of the solenoid.
There has also been the problem that in cases where the movable range is wide and the required propulsion in the controlled range is large, the size of the solenoid itself has to be increased to produce the propulsion.
For this reason, to solve the above problems, it is an object of the present invention to provide a solenoid that is small and where the propulsion within the controlled range can be increased.